1. Field of the Invention
The present invention relates to a fuel cell system, and more particularly, to a method for controlling peripheral system of a fuel cell and a fuel cell system using the same, wherein unexpected damages and adverse effects on lifetime and safety of an electricity storage device can be reduced (or minimized or prevented) by sequentially starting several peripheral devices for operating the fuel cell.
2. Discussion of Related Art
A fuel cell is a power generation system that directly converts chemical reaction energy into electric energy through an electrochemical reaction between hydrogen (or fuel) and oxygen (or oxidizing agent or oxidant).
According to the type of electrolyte used, the fuel cells can be categorized as a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid oxide fuel cell, a polymer electrolyte membrane fuel cell, an alkaline fuel cell, etc. These respective fuel cells are operated based on the same basic principle, but are different in the type of fuels used, operation temperatures, catalysts, electrolytes, etc.
As compared with other fuel cells, the polymer electrolyte membrane fuel cell (PEMFC) has relatively high output characteristic, a low operation temperature characteristic, and a rapid starting and responding characteristic. As such, the PEMFC is widely applicable as a mobile power source for a portable electronic equipment or a transportable power source for an automobile, as well as a distributed power source such as a stationary power plant for a house, a public building, etc. Also, a direct methanol fuel cell (DMFC) is similar to the PEMFC described above and can be considered as a type of PEMFC, but the DMFC can directly use a liquid methanol fuel. As such, since the DMFC does not need to use a reformer, the DMFC can be made smaller in size than the PEMFC needing a reformer.
A conventional PEMFC (or DMFC) system Includes a fuel tank for storing fuel, a fuel pump for transporting fuel, an air pump for compressing and transporting air, a controller for controlling a system, and a stack of unit cells. The stack (or fuel cell stack or fuel cell) has several tens to several hundreds of unit cells stacked adjacent to one another. A unit cell is composed of a membrane-electrode assembly (MEA) and a separator stacked on either side of the MEA. Herein, the membrane-electrode assembly has an anode electrode (also referred to as “fuel electrode” or “oxidation electrode”) and a cathode electrode (also referred to as “air electrode” or “reduction electrode”) that are adhered with each other with a polymer electrolyte membrane therebetween. The separator includes a flow channel for supplying a fuel (e.g., hydrogen) or an oxidizing agent (e.g., oxygen) and also function as a conductor to electrically connect a plurality of membrane electrode assemblies (MEAs) in series. In a system of the aforementioned construction, if a liquid fuel such as methanol aqueous solution or gas fuel such as hydrogen gas (or hydrogen-rich gas) is supplied to an anode of the stack, and an oxidizing agent such as oxygen gas, air in the atmosphere, etc., is supplied to a cathode, the stack generates electricity and heat by electrochemically reacting the fuel with the oxidizing agent.
Also, the conventional fuel cell system includes an electricity storage device having a controller for controlling a charge/discharge thereof, such as a secondary battery, a super capacitor, etc. When the fuel cell system starts, the electricity storage device supplies electric energy to the peripheral devices of the fuel cell such as the fuel pump, the air pump, fan, etc., or when a load demands excessive electric power, the electricity storage device supplies charged electric energy instead of energy from the fuel cell (or the stack of the fuel cell system) to the load. As such, the fuel cell system including the electricity storage device is capable of extending the time of use and optimizing the system response characteristics.
However, in the conventional fuel cell system, the electricity storage device is selected to smoothly supply starting electric power to the various peripheral devices of the fuel cell system installed for improving the operation efficiency of the system and to have sufficient capacity in order to meet the demanding power of the load when the load demands excessive electric power. In this case, as the weight and volume of the electricity storage device are increased, the weight and volume of the fuel cell system are also increased.
Also, the capacity of the electricity storage device is designed to be relatively large. That is, since the electricity storage device may be damaged by the maximum instantaneous electric power (i.e., instant peak electric power consumed in the peripheral devices when the fuel cell system starts), a relatively large electricity storage device is used to sustain (or account for) the instant peak electric power of the peripheral devices. However, in spite of this, the electricity storage device, such as a secondary battery, can still be (and/or is often) damaged by the instant peak electric power.
Furthermore, in the case of the fuel cell system using a secondary battery, when the remaining capacity of the secondary battery is small or the time required for starting is extended at the time of starting, over-discharge of the secondary battery is caused, which can damage or break the secondary battery. Therefore, in the conventional fuel cell system, the secondary battery having a power capacity larger than is needed for a typical operation of the fuel cell system should be used.
As described above, in the conventional fuel cell systems, the electricity storage device, such as the secondary battery or the super capacitor, can be damaged to detrimentally affect its safety, when the peripheral devices of the fuel cell start. As such, there is a need for a fuel cell system to have a scheme that does not have an adverse effect on the lifetime and/or the safety of an electricity storage device of the fuel cell system.